This invention relates to devices for receiving user input selections. More particularly, the invention concerns a circuit that responds to an increase in capacity as a result of a user contacting a conductive touch pad.
Switching circuits that respond to either the proximity of a user's limb or the physical contacting of a touch pad by a user, are desirable because the lack of mechanical movement avoids a source of failure. However, the cost for such touch-sensitive control circuits has tended to be prohibitive in all but the most expensive equipment because of the requirement for a regulated power supply and other compensating circuitry. While the supply of regulated power is ample in a high-end appliance incorporating, for example, a microprocessor, there exists a need for a touch control circuit that is capable of either energizing or deenergizing an electrical load in an application that does not otherwise require a regulated power supply. Not only does an "ON/OFF" system typically not require a regulated power supply to switch the load, the switching device that energizes and deenergizes the load may cause very large changes in the level of the supply voltage. In some applications, energizing the load may reduce the supply voltage by 50% or more. Accordingly, there is a need for a touch-responsive circuit that is substantially insensitive to very large swings in its supply voltage.
While circuits which alternatingly energize and deenergize a load typically respond to successive user touches of a single touch pad, other applications utilize multiple touch pads on a common substrate. For example, a numeric keypad and an input control circuit having discrete on and off touch pads are but two examples of multiple user selection means. Because the touch pads are located on a common substrate and cleaning fluid provides a path between pads, it has been necessary to manually disable the circuit whenever it is necessary to clean the substrate. Any user contact with a pad while cleaning the substrate would cause an ambiguous input condition in which multiple selections appear to be made at once. At other times, a user may inadvertently contact more than one touch pad and, hence, provide an ambiguous input condition to the touch circuit.